1. Field of the Invention
The present invention generally relates to metallurgical powder mixtures of the type comprising ferrous powder as a main constituent, wherein the ferrous powder is admixed with lesser amounts of alloying compounds, powdered lubricants or other additives as secondary components. In particular, the present invention relates to novel segregation-free compositions comprising such metallurgical powder mixtures which further contain polyvinyl pyrrolidone as a binder component in an amount sufficient to prevent dusting, lining or segregation of the powder components.
2. Brief Description of the Background Art
Processes for producing ferrous powders are well-known, as are many applications for these powders, such as powder metallurgy (P/M) part fabrication. For P/M applications, a ferrous powder is injected into a die cavity shaped to a desired configuration and a compact is formed of the material by the application of pressure. The compact is then sintered wherein metallurgical bonds are developed by the influence of heat. When necessary, secondary operations such as sizing, coining, repressing, impregnation, infiltration, heat or steam treatment, machining, joining, plating, etc. are performed on the P/M part.
It is a common practice to blend a lubricant together with the ferrous powder. This reduces friction between the pressed compact and the die walls during compaction which, in turn, lowers the required ejection force which is necessary to remove the compact from the die, lessening tool wear. Occasionally, the sintered materials which result from the P/M process may themselves be undesirable because, for example, the sintered forms may have insufficient parameters of physical "strength", i.e., rigidity or flexibility, hardness, tensile strength and the like. Thus, it is common to incorporate with the P/M iron powder minor amounts of at least one non-ferrous metal alloy powder to achieve desired physical properties in the final sintered product. Additionally, minor amounts of other additives may be utilized together with the ferrous powder to achieve the desired properties in the sintered product. The lubricants, alloying powders and other additives may be used together and are collectively referred to herein as "secondary powders".
Examples of this technology are found in various U.S. Pat. Nos. such as, for example, 2,888,738 to Taylor; 3,451,809 to Raman, et al.; 4,106,932 to Blachford; and 4,566,905 to Akashi, et al., as well as European patent application publication No. 0,266,936 to Larson, et al. and commonly-assigned U.S. Pat. No. 4,927,461 to Cilogluer, et al.
Although prior art P/M technology has thus been able to provide sintered materials with specific characteristics, and accordingly has been proven both technically and commercially successful, drawbacks still inherently plague the same. Namely, the present inventor has determined that if the P/M blends are to attain their desired performance characteristics, the powder blend must be maintained in a homogeneous admixture. Variations in the powder blend also contribute to inconsistencies in dimensional change. The secondary powders must not be allowed to migrate through the composition to the walls of the container holding the composition ("lining"), especially those secondary powders of higher density than the ferrous powder which, as a result of vibration, tend to migrate downwardly to settle on the bottom of the container. Also, the secondary powders which have a lower density than the ferrous powder cannot be permitted to migrate upwardly by air currents when being handled and conveyed ("dusting"). In doing so, the loss of homogeneity ("segregation") of the blend is prevented.
These problems can largely be ameliorated by judicious selection of constituents having appropriate specific gravities (see U.S. Pat. No. 4,504,441 to Kuyper). However, the physical properties of the secondary powders are generally of only secondary consideration to the primary goal of obtaining acceptable physical and metallurgical properties in the sintered end product. Therefore, overcoming dusting problems and the like by selecting powders with the goal only of obtaining specific densities has not proven to be highly successful.
Moreover, it is seen that dusting, lining or segregation problems are also exacerbated when the primary and secondary powders which are utilized in the composition are of significantly different sizes. However, those skilled in the art recognize that it is often necessary to utilize secondary powders of disparate size to the primary powders in order to resolve the conflicting requirements that (i) no primary powder particle be located further from a secondary powder particle than a predetermined number of primary particles and (ii) only a maximum amount of the secondary powders may be utilized in the powder blend (lest other physical properties of the sintered product be affected). That is, it is only possible to provide a sufficiently large number of secondary powder particles without increasing the weight amount of the secondary powder material by reducing the size of secondary powder particles.
However, reducing the secondary powder particle size may result in lining, dusting or segregation because the smaller secondary powder particles are physically excluded by the larger primary powder particles. Additionally, many secondary powders also have chemical characteristics or physical characteristics, such as shape, which encourage their segregation from the composition or indeed, even their aggregation. This is recognized, for example, in U.S. Pat. No. 4,676,831 to Engstrom which discusses the use of prealloyed powders. However, these prealloyed powders still fail to solve the problem of incorporating additional nonalloying materials such as the lubricants discussed above, or materials such as graphite.
A desirably homogeneous admixture of primary and secondary powders can be usually attained when the composition is first blended. Unfortunately, however, handling and conveying the blends leads to segregation of previously well-blended compositions.
One solution to these problems is to incorporate in the composition a third component to bind the secondary particles to the primary particles. Suitable binder components include sticky or viscous liquids such as oils, emulsions and the like (U.S. Pat. No. 4,676,831 to Engstrom). However, use of these materials is somewhat diminished because they tend to both make the powder composition agglomerate and inhibit its flowability.
Dry binder components have also been utilized, such as polyvinyl alcohol, polyethylene glycol, polyvinyl acetate (U.S. Pat. Nos. 3,846,126; 3,988,524 and 4,062,678 to Dreyer, et al., U.S. Pat. No. 4,834,800 to Semel).
Generally, thin liquid binders are homogeneously blended into the compositions and dried, while the viscous or powdery binders may be either blended dry (with dry or prewetted compositions), or dissolved in a carrier. Most commonly, however, viscous or sticky liquids are desirably dissolved in solvents to encourage homogeneous blending. Additionally, since it can be difficult to effectively blend dry binding components, they are usually first dissolved in solvent, dispersed throughout the powder blend, whereupon the solvent is evaporated.
Although solid and viscous binders can be dispersed when they are dissolved in solution, competing problems of making the solution thin enough to disperse well versus minimizing the amount of diluent used (since it later needs to be evaporated) provides that only a relatively narrow range of solution concentration is desired. Inasmuch as it may be difficult to determine the optimal amount of solvent, it has been known (see U.S. Pat. No. 4,504,441 to Kuyper) to mix a quantity of liquid furfuryl alcohol into a powder composition and then blend in an acid to polymerize and solidify the furfuryl alcohol. However, the present inventor has determined that the use of solid binders, such as Kuyper's polymerized compound increases the compacting pressure which is needed to densify the metallurgical blends.
It is also said that the use of water-soluble binders is disadvantageous since they may be difficult to dry, absorb moisture and encourage rust. Therefore, those of ordinary skill in the art prefer to utilize polymeric binding agent resins which are water-insoluble or substantially water-insoluble, such as polyvinyl acetate, polymethacrylate, or cellulose, alkyd, polyurethane or polyester resins (U.S. Pat. No. 4,834,800 to Semel).
The present invention addresses and overcomes many of the deficiencies of the prior art by providing a novel metallurgical powder blend comprising a binder of polyvinyl pyrrolidone. These features and others are provided by a metallurgical powder composition comprising ferrous powder having a maximum particle size of at most about 300 microns; and at least one of (i) an alloying powder in the amount of less than about 15 weight percent, (ii) a lubricant in the amount of less than about 5 weight percent and (iii) an additive in the amount of less than about 5 weight percent, said composition further comprising a binding agent for preventing the alloying powder or lubricant from segregating from said composition, said binding agent comprising polyvinyl pyrrolidone.